Getting those first forum members is not easy, but it can be done. Here are some tactics I have used over the years to boost activity on a brand new community:
1. have the forum structure ready - don't go crazy with a ton of categories, but do create a list of sub-topics that you will talk about. Having one big-Bertha category where everything should fit it makes the forum look empty and badly organized. Do not yet created sub-sub-categories (example: I have "medical seo" for instance, I haven't yet created "local seo" or "seo tools" sub-sub categories to keep the forum still clean).
2. start with 30-40 threads - have a few threads in each category, this way your new members will be enticed to reply. New people are not always that "brave" to start threads, so they'll most likely reply, while testing the waters.
3. have a "say hello" section and welcome new members - it allows new community members to feel welcome and see how warm the atmosphere is.
You can also send a private message, as soon as they have approved their account (it's a simple setting), letting them know about the main rules and encouraging to start posting in the community.